1. Field of the Invention
The invention concerns brushes for commutated DC motors in which the contact between brush and commutator can be programmed to change position as the brush wears.
2. Description of Related Art
FIG. 1 illustrates a generalized commutator 2 in a simple DC machine. Commutator contact 3 (shown hatched) and contact 6 (not hatched) connect to leads 9 and 12 of a coil 15, respectively. Brushes 18 (of positive polarity, as indicated) and 21 (of negative polarity) deliver current to the contacts, and cause current 24 to flow in the coil 15.
When the commutator 2 is in the position shown in FIG. 1A, the current 24 flows in the direction shown, with respect to reference dot 27, which is considered fixed to the coil 15. When the commutator 2 rotates to the position shown in FIG. 1B, the current 24 reverses in direction, with respect to the dot 27 (although, of course, the current still flows from the positive brush 18 to the negative brush 21).
Therefore, rotation of the commutator 2 causes current within the coil 15 to reverse direction. This reversal causes reversal of the magnetic field lines 30 with respect to the coil 15, as indicated, which are generated by the current 24. Even though the magnetic field lines 30 point leftward in both FIGS. 1A and 1B, it should be remembered that, in FIG. 1B, the coil 15 is inverted, with respect to FIG. 1A. Consequently, the magnetic field lines 30 have become reversed, in FIG. 1B, with respect to the coil 15.
The magnetic field lines interact with a stationary magnetic field 30S, produced by a stator (not shown). Since the two magnetic fields 30 and 30S want to align with each other, they urge the coil to rotate, in order to allow the alignment. However, since the field lines 30 associated with the coil 15 continually reverse in direction, the coil 15 continually rotates in pursuit of this alignment. (If a non-changing, DC current flowed in the coil 15, the coil would stop rotating once the fields became aligned.)
FIG. 2 is an enlarged view of brush 18 of FIG. 1. As the brush 18 wears, and material is removed by the wear, a spring (not shown) causes the brush 18 to advance in the direction of arrow 19. The brush 18 advances along a reference line 33, drawn exactly at the 12 o'clock position. During this advancement, the center 36 of the region of contact remains fixed on reference line 33, as indicated in FIGS. 2B and 2C.
In a DC motor generally, changing the position of point 36 changes the speed, or torque, or both, produced by the motor. It can be desirable to change the position of point 36 during the lifetime of the motor, for various purposes.
One purpose is to compensate for changes in speed which are caused by wear. For example, when the brush configuration changes from that of FIG. 2A to FIG. 2B, the area of contact becomes larger, and the average time during which brush 18 shorts two or more adjacent commutator segments, and therefore two or more armature coils, increases. This change in contact area can change motor speed. It may be desirable to move point 36, in an attempt to counteract the change in motor speed.